The TGF-β superfamily consists of more than 25 different signaling proteins found in both vertebrates and invertebrates. Protein members of the TGF-β superfamily influence a wide range of biological processes including cell growth, cell growth inhibition, tissue repair, cell differentiation, apoptosis, establishment of the dorso-ventral embryo body axis, and secretion of extracellular matrix components (Ebendal et al. 1998, Journal of Neuroscience 51:139). Thus, members of the TGF-β superfamily provide attractive targets for the development of pharmaceutical agonists and antagonists that may be used to treat a wide variety of human diseases and conditions which are affected by the activity of one or more of the superfamily members. For example, antagonists and agonists of TGF-β superfamily members have practical application in the area of tissue repair and regeneration as well as in the area of differentiation of pluripotent stem cells into cells or tissues of a preferred lineage. Additionally, TGF-β superfamily members also provide targets for gene therapy. The cloning and expression of many members of the family has been described. (See e.g. U.S. Pat. Nos. 4,877,864; 5,108,922; 5,013,649; 5,116,738; 5,106,748; 5,187,076; 5,141,905; 5,688,678; 5,661,007; 5,637,480; 5,639,638; 5,658,882; and 5,635,372).
Among the members of the TGF-β superfamily are the bone morphogenetic proteins (BMP). BMPs were initially identified as regulators of cartilage and bone formation. Subsequent work has shown that BMPs, like other TGF-β superfamily members, play a role in many different biological processes including embryogenesis and morphogenesis of a variety of organs and tissues. Additionally, BMPs play a role in the growth, differentiation, and chemotaxis of several different cell types such as hematopoietic cells, epithelial cells, mesenchymal cells, and neuronal cells (Reddi, 1998, Nature Biotechnology 16:247; Ebendal, supra).
BMPs, like other members of the TGF-β superfamily are highly conserved across different animal species. The mature human BMP-2, for example, is completely homologous with mouse and rat BMP-2. The biologically active form of BMP-2 is a homodimer consisting of a disulfide linked carboxy terminal domain of 114 amino acids. BMP-2 exerts its affect on target cells by binding to a cell surface receptor comprised of a hetero-oligomer. The receptor is a complex of two serine/threonine kinase receptors (see Ebendal, supra; Reddi supra).
The human homolog of BMP-2 has been cloned, Wozney, 1989, Prog. Growth Factor Res. 1(4):267. Recombinant human BMP-2 can be expressed as a fragment of the full length BMP-2 consisting of either amino acids 266–396 or 283–396. The fragments form both homodimers and heterodimers resulting in six different isoforms. The six dimeric isoforms are denoted: <Q283/<Q283, <Q283/Q283, Q283/Q283, <Q283/T266, Q283/T266 and T266/T266 and can be separated by cation exchange chromatography (FIG. 1). The number 283 or 266 refers to the N-terminal amino acid position in the full length rhBMP-2. The letter refers to the amino acid at the N-terminus (i.e. Q or T) and the “<” refers to the cyclization of the glutamine (Q) at position 283 to form pyroglutamic acid. Thus, for example, <Q283/Q283 refers to a rhBMP-2 disulfide linked dimer wherein one monomer of the dimer has the cyclized N-terminal glutamine and the other monomer does not.
rhBMP-2 has several clinical applications which require the regeneration of bone. Thus, rhBMP-2 can be used for spinal fusion to treat degenerative disk disease. rhBMP-2 can also be used to treat long bone fractures. It can also be used to treat individuals requiring artificial teeth who lack sufficient bone mass to support the implants.
Maintaining the solubility of a protein is frequently important either for maintaining bio-availability and/or activity of the protein. Protein solubility is dependent upon a variety of factors. These factors include environmental conditions such as pH, salt concentration, temperature and the chemical characteristics of the solvent, as well as innate properties of the protein of interest, such as the primary amino acid sequence and the structural conformation of the protein. Frequently, bio-medical and/or pharmacological applications involving a protein of interest require environmental conditions which do not optimize the solubility of the protein of interest. Precipitation of the protein is the result, thus limiting bio-availability and/or activity of the protein of interest. Accordingly one aspect of the invention relates to increasing the solubility of a protein of interest, e.g., a member of the TGF-β superfamily, and thereby increasing its bio-availability and/or activity either as a pharmaceutical composition or a research reagent.
The instant invention is based at least in part on the surprising discovery that various peptide fragments of recombinant human BMP-2 (rhBMP-2) increase solubility and/or inhibit precipitation, of a protein, such as, a member of the TGF-β superfamily.